This invention relates generally to a compressible treadle switch and assemblies containing the same which are installed in roadways for sensing vehicular traffic flow, for counting the number of axles on individual vehicles for toll assessment purposes, and the like. More specifically, the invention relates to such treadles which, when compressed, interrupt a light beam generated by an associated optical system to thereby operate a vehicle traffic counter and/or vehicle axle counter.
Broadly speaking, compressible treadles which are disposed in a rigid tray or frame buried in a surface portion of a highway and which operate, when compressed, to close the contacts of an electrical switch to actuate an electrical counter for counting the number of vehicles passing thereover and/or for counting the number of axles on individual vehicles, have long been known and used in the prior art. See, for example, the treadle assembly disclosed in U.S. Pat. No. 4,455,456 issued to D. Habeger on Jun. 19, 1984. The reference system includes a plurality of elongated treadles of generally trapezoidal cross-section which are formed of a compressible, resilient material such as natural or synthetic rubber, polymer plastic or the like. These treadles are disposed within a steel tray in parallel extending, spaced apart relationship. Elongated retaining bars mounted on base plates and affixed thereto with countersunk hex nuts are disposed between the treadles to retain the latter within tray.
The tray containing the aforesaid components is then mounted on a support frame and buried within a roadway so that the tires of vehicles passing along the roadway will roll across the exposed treadles and compress the same. Each treadle defines a longitudinally extending hollow passageway therethrough containing a pair of elongated metallic, electrical contact strips attached to upper and lower surfaces of the passageway. As the tires of a vehicle cross a given one of the treadles, the treadle material is vertically compressed, causing the strips on the upper and lower surfaces of the passageway to be pressed together to thus close an electrical circuit and operate a counter. The reference treadle assembly thus operates as an electrical switch to control a counter in response to a compression force applied to the treadle by a vehicle tire.
Another compressible treadle switch assembly for use in a roadway is disclosed in U.S. Pat. No. 2,885,508 issued to H. A. Wilcox on May 5, 1959. As in the previously mentioned patent, the instant treadle contains a longitudinally extending passageway having elongated, spaced apart electrical contact strips attached to upper and lower defining surfaces of the passageway. Vehicle tires rolling across the subject treadle compress the same to bring the strips into contact with one another to actuate an electrical counting circuit.
Care must be exercised in the shipping and handling of these prior art treadles so as not to bend them and thus permanently deform and mechanically short circuit the contact strips. Because of this problem, it is wise to securely pack such treadles in rigid crates for shipping purposes. It is also essential that the open ends of the passageways of such treadles be carefully sealed to prevent the introduction of excessive moisture therein which could cause the electrical contact strips to short circuit. It is difficult to obtain an effective moisture seal in most cases, by reason of which these prior art treadles tend to permanently short out between the terminal strips after a relatively short period of use. On the average, such treadles must be replaced after only about six months of use due to excessive moisture in the passageway which has produced a permanent shorting of the contact strips. Finally, because of the care required in the handling of such treadles prior to and during installation so as to avoid permanently bending the contact strips, a great deal of time must be taken in order to carefully install them.
By means of my invention, these and other difficulties encountered using prior art treadles and assemblies containing the same are substantially overcome.